(1) Field of the Invention
This invention relates to the surface treatment of copper and its alloys, and more particularly to coatings providing tarnish resistance and/or improved package adhesion for integrated circuit lead frames.
(2) Description of the Related Art
U.S. Pat. No. 5,449,951 of Parthasarathi and Mahulikar (the '951 patent) discloses the use of a chromium-zinc coating on the lead frame of an integrated circuit (IC) package. The disclosure of the '951 patent is incorporated herein by reference as if set forth at length. The coating of the '951 patent, when applied to a copper lead frame, improves the adhesion between the lead frame and the polymeric material of the IC package. The '951 patent teaches electrolytic application of the coating from a solution comprising excess hydroxyl ions (OH), hexavalent chromium ions (Cr(VI)) and zinc ions. The '951 patent references the electrolytic deposition of U.S. Pat. No. 5,022,968 of Lin and Chao (the '968 patent). The '968 patent discloses use of sodium hydroxide or potassium hydroxide as the hydroxyl ion source.
The '951 patent identifies plating the lead frame prior to attachment of the semiconductor die to the lead frame die pad (a.k.a., die attach pad or paddle). After attachment, bond pads on the die are wire bonded to associated leads of the lead frame. To facilitate such wire bonding, the associated inboard areas of the leads are masked prior to being coated. Prior to the wire bonding, the mask is removed, permitting direct contact between each bond wire and the associated lead. Alternatively, the coating can be applied over the entire lead and, thereafter, etched or otherwise removed at the wire bonding location.
Contamination from ionic species is believed associated with die failure. See “Metallization Contamination,” Microelectronic Defects Database, CALCE Electronic Products and Systems Center, University of Maryland, Apr. 12, 2000 (identifying chloride and sodium contamination from molding compounds, die coatings, and die attach coatings) and Barnes and Robinson, “The Impact of Ionic Impurities in Die Attach Adhesives on Device Performance”, Proc. of 34th Electronics Components Conf, May, 1984, p.68 (identifying die attach adhesives as a particular source of ionic contamination).